Free radical polymerization of ethylene, with or without the presence of comonomers, produces low density polyethylene (LDPE). There are two basic processes for the manufacture of LDPE: autoclave and tubular. The LDPE made by the autoclave reactor process (“autoclave LDPE”) has a high concentration of long chain branches that makes it easy to process. However, the autoclave reactor process has a low monomer conversion rate, usually about 20%. The tubular reactor process has a significantly higher monomer conversion rate than the autoclave reactor process. It has a monomer conversion rate up to 40%. The tubular reactor process requires a lower capital cost per unit of production than the autoclave process.
The LDPE made by the tubular reactor process (“tubular LDPE”) has a lower concentration of long chain branches than the autoclave LDPE. The autoclave LDPE shows smaller g′-parameter, the ratio of intrinsic viscosity to that of linear polyethylene. Further, the autoclave LDPE exhibits larger upturn behavior than the tubular LDPE. In the stress relaxation measurement, the autoclave LDPE shows a larger damping function, while the tubular LDPE shows values similar to those predicted by the Doi-Edwards theory. Furthermore, there is a significant difference in the effect of the shear history on the melt strength. The melt strength of the autoclave LDPE decreases more rapidly with the shear history than that of the tubular LDPE. All of these differences between the two types of LDPEs are due to the difference in the branch structure. The complex, multi-branched structure of the autoclave LDPE gives much more prominent elastic features. See Rheological Properties of Low-Density Polyethylenes produced by Tubular and Vessel Processes, Yamaguchi Masayuki, and Takahashi Masaoki, Polymer 2001, Vol. 42, No. 21, pp. 8663-8670.
Although the tubular LDPE resins are good for general purpose films and cables, they are not suitable for some applications in which the autoclave LDPE resins are used. For instance, the tubular LDPE resins are not suitable for extrusion coatings because they give poor drawdown and high necking. Methods for modifying tubular LDPE are thus needed. Ideally, the modification would increase the long chain branching of the tubular LDPE to a similar level to the autoclave LDPE.